I know I've posted this before, but I've been using FreeNAS and it's a great use for an old PC. I'd recommend it if you want to toy around, not for extremely important data. It'll support almost any data protocol you could want, and there are many other things you can do with it after you have it configured.
FreeNAS: The Free NAS Server - Home
FreeNAS: The Free NAS Server - Home